Hydraulic system on B 737 divided into 3 system which is A, B & Standby however only A & B system quantity shown in the system page on lower display unit. Standby system reservoir is connected to system B.

It can happen that few amount of hydraulic fluid did transfered to one another during flight even though theoretically it's not posible. It can happen through movement of shuttle valve that connect both system together when it supplied as back-up such as alternate brake. To normalise it back basically we need to drain it and replenish another system system with new fluid.

Experienced 737 engineer know the trick to transfer it with-out to drain and topping-up. Here i would like to share you the trick. From the photo we can see system B is higher, now to transfer from B to A.

1. Turn on B emdp pump.2. Set park brake.3. Turn on A emdp pump.4. Turn off system B pressure by operate the flight control5. Release park brake.6. Turn off system A emdp pump.

After few repetition we can see the fluid transfered to another.

* This is just for information sharing and not to replace the aircraft maintenance manual procedure.